Abstract More than half of Emergency Medical Services (EMS) workers report work-related mental and physical fatigue. Odds of injury among fatigued EMS workers ? most of which include musculoskeletal injuries and exposure to blood borne pathogens ? are nearly double that of non-fatigued workers. There is a compelling need to reduce fatigue among EMS workers, yet few EMS organizations have a formal fatigue management program and many may not be cost-effective or evidence-based. Recent research supports prompting EMS workers in real- time with mobile phone text-messages to adopt alertness promoting behaviors as a strategy to manage work- related fatigue. We recently completed two randomized trials and demonstrated feasibility and impact on work- related fatigue using real-time text-message prompts (Clinicaltrials.gov NCT02063737 & NCT02783027). Our overarching goals are to 1) help EMS workers reduce fatigue at work, reduce fatigue-related injury, and improve sleep health; and 2) improve EMS organization-level fatigue risk management. Our global hypothesis is that dissemination of tailored sleep health information combined with tailored alertness promoting strategies in real-time can improve intra-shift fatigue, sleep health, and ultimately workplace safety culture. We base our hypothesis on our recent pilot trial (SleepTrackTXT). In that trial, 99 of 100 enrolled EMS workers participated for 90 days and reported sleepiness and fatigue in real-time before, during, and after scheduled shifts. Workers prospectively documented 2,621 shifts and responded to 36,073 text-messages in real-time. Compliance with answering text-messages was 88.1%, and end of shift fatigue was lower in the intervention group that worked 12-hour extended shifts compared to control subjects (p=0.0004). Preliminary analyses of our second trial show findings similar to our first trial. Participants responded to 61,571 of 69,530 text-message queries (an 89% response rate; Unpublished Data), and end of shift fatigue was lower at the end of 12-hour shifts in the intervention group compared to the control group. We seek to test effectiveness of an enhanced version of our SleepTrackTXT intervention on work-related fatigue and sleep health indicators. Aim 1: Determine the impact of the enhanced intervention on EMS worker fatigue and sleep health in a cluster-randomized trial. Aim 2: Identify worker-level and EMS agency-level conditions linked to a reduction in worker fatigue. Aim 3: Perform exploratory analyses to determine impact of the enhanced intervention on the incidence of work-related injury and workplace safety culture. Significance: This proposal addresses fatigued EMS workers, for which there has been no standard for fatigue risk management. This proposal logically builds on our pilot trial, which demonstrated feasibility, high compliance, and a positive impact on indicators of sleep and fatigue. This proposal addresses national goals of the National Occupational Research Agenda (NORA) and tests a novel approach to fatigue risk management that is easily scalable to large workforces and low-cost for employers of shift workers.